A first implementation of ItemOwnershipPolicy extension

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Migrated OwnershipPlugin's configuration to the real class instance

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #21 from jenkinsci/item-ownership-policy

[JENKINS-21904] - ItemOwnershipPolicy extension point